Current sensors are known to those of ordinary skill in the art. One type of conventional current sensor uses a magnetic field sensing element (for example a Hall effect element or magnetoresistance element) in proximity to an electrical conductor. The magnetic field sensing element generates an output signal having a magnitude proportional to the magnetic field induced by an electrical current that flows through the electrical conductor. Therefore, it will be understood that the current sensor senses a magnetic field associated with the electrical current.
Some typical current sensors include a gapped toroid magnetic flux concentrator, with the magnetic field sensing element positioned in the toroid gap. The magnetic field sensing element and toroid are assembled into a housing, which is mountable on a printed circuit board. In use, an electrical conductor, such as a wire, is passed through the center of the toroid. The toroid acts as a magnetic flux concentrator, providing an increased magnetic field through the magnetic field sensing element, and therefore, a more sensitive device. However, such devices tend to be undesirably large, both in terms of height and circuit board area.
Proximity detectors (also referred to herein as rotation detectors) for detecting ferromagnetic or magnetic objects are also known. One application for such devices is in detecting the approach and retreat of each tooth of a rotating ferromagnetic object, such as a ferromagnetic gear. The magnetic field associated with the ferromagnetic object is often detected by one or more magnetic field sensing elements, such as Hall elements or magnetoresistance elements, which provide a signal proportional to a detected magnetic field (i.e., a magnetic field signal). The proximity detector processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses a threshold. Therefore, when the proximity detector is used to detect the approach and retreat of each tooth of a rotating ferromagnetic gear, the output signal is a square wave representative of rotation of the ferromagnetic gear. It will be understood that the proximity detector, like the current sensor described above, senses a magnetic field. The proximity detector senses a magnetic field associated, for example, with the gear teeth.
Magnetic field sensors are also known. Like the current sensor and the proximity detector described above, one type of magnetic field sensor uses a magnetic field sensing element (for example a Hall effect element or magnetoresistance element) in the presence of a magnetic field. The magnetic field sensing element generates an output signal having a magnitude proportional to the magnetic field. It will be understood that the magnetic field sensor, like the proximity detector and the current sensor, senses a magnetic field.
Various parameters characterize the performance of current sensors, proximity detectors, and magnetic field sensors, including sensitivity and linearity. Sensitivity is related to the magnitude of a change in the output from the magnetic field sensing element used in the current sensor, proximity detector, or magnetic field sensor in response to a sensed current, a sensed ferromagnetic object, or a magnetic field, respectively. Linearity is related to the degree to which the output of the magnetic field sensing element varies in direct proportion to the sensed current, the sensed ferromagnetic object, or the sensed magnetic field, respectively. One of ordinary skill in the art will recognize that above-described output can either be in the form of a voltage output or a current output.
The sensitivity of the current sensor, the proximity detector, and the magnetic field sensor is related to a variety of factors. One important factor is the magnitude of the sensed magnetic field. For this reason, some current sensors, proximity detectors, and magnetic field sensors use a flux concentrator disposed in order to concentrate a magnetic flux and to direct the concentrated magnetic flux through the magnetic field sensing element.
It will be understood that the amount of sensitivity improvement provided by a magnetic flux concentrator is related to a separation between the magnetic flux concentrator and the magnetic field sensing element. It will also be understood that both sensitivity and linearity are reduced if the magnetic flux concentrator becomes magnetically saturated.
A prior art approach to provide a magnetic flux concentrator in proximity to a magnetic field sensing element is described in Fully Packaged CMOS Integrated Current Monitor Using Lead-On-Chip Technology, R. Steiner et al., 0-7803-4412-X/98, IEEE, 1998. In this approach a bottom surface of a substrate is etched under and proximate to a magnetic field sensing element disposed on the top surface of the substrate. The etching provides a V-shaped channel into which a soft magnetic material is disposed, resulting in a magnetic flux concentrator close to the magnetic field sensing element. The resulting V-shaped magnetic field concentrator, though close to the magnetic field sensing element, tends to saturate at the apex of the channel, due to the V-shape. As described above, the saturation tends to reduce linearity in the magnetic field sensing element.